Humans have lived alongside animals for a very long period. So long, in fact, that it is hard to imagine a situation where we would not have a furry or feathered companion by our side or nearby. It should be noted that the number of domesticated species may vary depending on the chosen definition of "domestication". Scientists have identified 30–40 species that have been domesticated or with which mutually beneficial "contacts" have been established. The criteria for domestication do not suggest that this process was simple or linear; it began and evolved over thousands of years, at different times and in different places on Earth. It must be emphasised that by domesticating animals, humans changed them to a certain extent, and, in turn, domesticated animals also changed humans.